Process for purifying syngas

ABSTRACT

The present invention relates to a process for the purification of raw syngas comprising the steps of (a) contacting the raw syngas with water to remove soot, resulting in a soot-rich waste water and a soot-lean syngas; (b) cooling the soot-lean syngas resulting from step (a) to a temperature in the range of from (20) to (50)° C.; and (c) contacting the cooled soot-lean syngas resulting from step (b) with water to remove HCN and NH3 resulting in clean syngas and HCN/NH3-rich waste water, wherein: (i) in step (a) the raw syngas is cooled below its dew point by its contact with water; (ii) the HCN/NH3-rich waste water resulting from step (c) is passed through at least one stripping column resulting in a clean water stream and a waste stream; and (iii) at least 50% (v/v) part of the clean water stream is recycled to step (c).

FIELD OF THE INVENTION

The present invention relates to process for the purification for syngas. The invention also relates to a clean syngas obtained by said process.

BACKGROUND OF THE INVENTION

The expression “syngas” as used herein refers to synthesis gas, which is a common term to refer to gas mixtures comprising carbon monoxide and hydrogen.

Processes for the preparation of syngas are well known in the art. Typically a feed gas comprising methane is contacted with an oxidizing gas and the methane reacts with the oxidizing gas to form a syngas. Syngas produced by such known gasification processes contains impurities, in particular soot and nitrogenous impurities, such as ammonia (NH3) and hydrogen cyanide (HCN). Syngas can be used in a variety of chemical processes, in which it is converted in a desired product. Usually such conversion is a catalytic process, for example a Fischer-Tropsch process. Catalysts used in those conversion processes are often very sensitive to certain impurities in the syngas which would cause fouling (soot) and/or poisoning (NH3, HCN) of the catalyst used. Deactivation of the catalyst and selectivity loss, often irreversible, will occur as a result. Accordingly, it is important that impurities, such as soot, NH3 and HCN, are effectively removed from the syngas before using the syngas as feed in a catalytic conversion process.

Methods and devices for removing soot and/or NH3 and HCN from syngas are known in the art. For example, according to WO-2008/155305-A the NH3 and HCN are removed from the syngas by hydrolysis of HCN to NH3 at elevated temperature followed by passing the hydrolysed syngas over an acidic cation exchange resin in the presence of water to remove NH3.

WO-99/38795-A1 discloses a process for producing and cleaning a syngas. In this process most of the HCN is catalytically converted into NH3 which, along with some of the HCN, is subsequently removed from the syngas with water to form an aqueous solution of NH3 and HCN. The hydrocarbon gas feed to the synthesis gas generator is then used to strip NH3 and HCN from the aqueous solution of NH3 and HCN that was formed. In the syngas generator the NH3 and HCN are consumed to form clean water. A portion of the resulting clean water is recycled back into the process where it is used to scrub the synthesis gas, with the remainder used for other purposes or sent to disposal.

U.S. Pat. No. 4,189,307 discloses a process for producing clean HCN-free syngas from raw syngas leaving a partial-oxidation gas generator by a continuous process comprising the steps of partial cooling, scrubbing with condensate, cooling below the dew point by indirect heat exchange preferably with a rich liquid absorbent from a downstream acid-gas-removal zone, and scrubbing with cold aqueous absorbent. The HCN-containing aqueous absorbent resulting from this scrubbing step is then processed, for example, by stripping it or by reacting it in the gas generator. Optionally, other acid gases, if present, may be removed from the synthesis gas in said acid-gas-removal zone.

US-2012/0202897-A1 discloses a method for removing hydrogen sulphide and other impurities, such as NH3, COS, HCN and small alkali metal compounds from syngas obtained from gasification of a biomass feedstock. The method comprises contacting the syngas with an aqueous absorbent containing low levels of certain metal ions to absorb the impurities. US-2012/0202897-A1 refers in particular to sulphur compounds as impurities to be removed: the absorbed sulphur compound reacts with the metal ions in the aqueous absorbent to form metal sulphide precipitates. These precipitates can subsequently be removed from the absorbent by e.g. filtration. In one embodiment this method is carried out in a single vessel comprising three successive spray sections separated by sieve plates. The syngas enters the column via an inlet distribution in the bottom part and is countercurrently contacted with the aqueous absorbent containing the metal ions in the successive spray sections.

The process line-up for removing soot and NH3/HCN (or other contaminents) from syngas as described in the prior art require the heavy import of water and any other solvent, for example a solution containing ammonium polysulfide, for removing the soot and NH3/HCN. In addition, the process line-up as described in the prior art require a large number of equipment and complex heat integration because of the different operating temperature of all the processing steps and/or the use of catalytic processes. The catalytic processes require catalysts, frequent change-outs and subsequent regeneration—all sum up to higher operations costs. Also, the prior art discloses processes which do not guarantee the soot removal up to the required specification of nill.

It is an object of the invention to provide an effective process for the effective removal of soot, NH3 and HCN without the requirement of heavy import of water or any other solvent. It is a further object of the invention to provide a process with lesser process steps, and therefore smaller number of equipment. Another object is to provide a process without the dependency on catalyst and resins performance and change-outs.

SUMMARY OF THE INVENTION

From a first aspect, above and other objects may be achieved according to the present invention by providing a process for the purification of raw syngas comprising the steps of

-   (a) contacting the raw syngas with water to remove soot, resulting     in a soot-rich waste water and a soot-lean syngas; -   (b) cooling the soot-lean syngas resulting from step (a) to a     temperature in the range of from 20 to 50° C.; and -   (c) contacting the cooled soot-lean syngas resulting from step (b)     with water to remove HCN and NH3 resulting in clean syngas and     HCN/NH3-rich waste water,     wherein: -   (i) in step (a) the raw syngas is cooled below its dew point by its     contact with water; -   (ii) the HCN/NH3-rich waste water resulting from step (c) is passed     through at least one stripping column resulting in a clean water     stream and a waste stream; and -   (iii) at least 50% (v/v) part of the clean water stream is recycled     to step (c).     It has been found that the process according to the present     invention no import of water or any other solvent is necessary to     purify the syngas. The water supply is condensed water from syngas.

The invention also relates to a clean syngas obtainable according to a process according to the present invention.

An advantage is that the purified syngas can be used in catalytic conversion processes without negatively effecting the catalyst.

DETAILED DESCRIPTION OF THE INVENTION

In step (a) of the process according to the present invention the raw syngas is contacted with water to remove soot, resulting in a soot-rich waste water and a soot-lean syngas. Suitably, raw syngas is contacted with water in step (a) at a temperature range of from 120 to 160° C., preferably at a range of from 130 to 160° C. and at a pressure in the range of from 35 to 80 bar, preferably 45 to 60 bar. The amount of clean water added at the start-up of the plant in step (a) can be neglected in view of the the amount of water generated in the present process, for example the condensed water from syngas, recycled to step (a).

By the term “raw” in “raw syngas” is meant syngas containing impurities, in particular soot and nitrogenous impurities, such as ammonia (NH3) and hydrogen cyanide (HCN). The amount of soot in the raw syngas in in a range between 25 and 65% (v/v). The temperature of the raw syngas in step (a) is in a range of from 110 to 220° C., preferably 120 to 180° C. In step (a) the raw syngas is cooled below its dew point by its contact with water. This water may be clean water, condensed water from syngas and/or recycled water. Preferably, the water added in step (a) originates from condensed water from syngas. The temperature of water in step (a) is between 10 and 60° C., preferably between 20 to 50° C. The heat contained in the raw syngas is effectively transferred to the water (i.e. cooling of the raw syngas) and any soot particles contained in the raw syngas can be effectively captured by water droplets condensing on such soot particles. The dew point of a typical raw syngas is between 140 and 150° C.

In step (a) a soot-rich waste water and a soot-lean syngas is obtained. Suitably, the amount of soot in the soot-rich waste water is in the range of 25 to 65% (v/v), based on the total amount of soot in the raw syngas. With soot-lean syngas is meant that the soot content in the syngas is virtually nill. Preferably, the raw gas obtained in step (a) is soot-free syngas.

Preferably, the temperature of the soot-rich waste water obtained in step (a) is between 120 and 160° C., preferably 120° C. A first part of the soot-rich waste water resulting from step (a) is preferably cooled and re-used in step (a).

Typically, the soot-rich waste water is cooled by a heat exchanger, such as an air and/or water cooler. Preferably, part of the colled soot-rich waste water is recycled to step (a). Suitably, a second part of the soot-rich waste water is cleaned before further treatment. Preferably between 60 and 90% (v/v), more preferably between 70 to 85% (v/v), even more preferably 83% (v/v), of the soot-rich waste water is cleaned before further treatment.

The soot-rich waste water is preferably cleaned from absorbed contaminants in at least one waste water stripper. Preferably, the cleaning step of the second part of the soot-rich waste water is at a pressure between 3 and 5 bar and at a temperature between 130 and 160° C.

The temperature of the soot-lean syngas will usually be between 40 and 80° C. lower than the temperature of the raw syngas and will be in the range of from 100 to 140° C.

In step (b) of the process according to the present invention the soot-lean syngas resulting from step (a) is cooled to a temperature in the range of from 20 to 50° C. The soot-lean syngas in step (b) is preferably cooled by a series of heat exchangers, such as air and/or water coolers.

In step (c) of the process according to the present invention the cooled soot-lean syngas resulting from step (b) is contacted with water to remove HCN and NH3 resulting in clean syngas and HCN/NH3-rich waste water. Preferably, the cooled soot-lean syngas from step (b) is contacted with water at a temperature range of from 20 to 50° C. and a pressure between 40 to 60 bar. Suitably, the water used in step (c) has a temperature between 10 and 60° C., preferably 20 to 50° C. The amount of HCN and NH3 in soot-lean syngas is between 10 and 100 ppbv based on the total amount of HCN and NH3 in raw syngas. The amount of HCN and NH3 in the clean syngas is virtually nihil.

The HCN/NH3-rich waste water resulting from step (c) is passed through at least one stripping column resulting in clean water stream and a waste stream. The waste water stream may comprise from 10 to 100 ppbv of HCN and NH3. Preferably, at least 50% (v/v) part of the clean water stream is recycled to step (c). More preferably, at most 90% (v/V) of the clean water stream obtained from the HCN/NH3 rich waste water stream is recycled to step (c). Also, at most 10% (v/v) of the clean water stream obtained from the HCN/NH3 rich waste water stream is recycled to step (a)

In a further aspect, the present invention provides a clean syngas. The clean syngas preferably contains no soot (virtually nihil) and 10 to 100 ppbv of HCN and NH3 based on the total amount of soot, HCN and NH3 in the raw syngas.

The Figure schematically shows a process scheme of a preferred embodiment of the process according to the present invention.

For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line.

The process scheme is generally referred to with reference numeral 1.

A raw syngas 10 enters a soot scrubber 2 to obtain a soot-rich waste water stream 20 and a soot-lean syngas 30. The soot-rich waste water stream 20 is fed to a water stripper 3 to obtain a clean water stream 40 for further treatment. Part of stream 20 is recycled to soot scrubber 2. The soot-lean syngas 30 is cooled with a heat exchanger 4 and the cooled soot-lean syngas is fed into a HCN/NH3 scrubber 5 to obtain a purified clean syngas 60 and a HCN/NH3-rich waste water stream 70. The HCN/NH3 rich waste water stream 70 is fed into a HCN/NH3 stripper 6 to obtain a clean water stream 80 and a waste stream 90. The clean water stream 80 is recycled back to the HCN/NH3 scrubber 5. Part of the clean water stream 80 is recycled back to the scrubber 2. 

1. Process for the purification of raw syngas comprising the steps of (a) contacting the raw syngas with water to remove soot, resulting in a soot-rich waste water and a soot-lean syngas; (b) cooling the soot-lean syngas resulting from step (a) to a temperature in the range of from 20 to 50° C.; and (c) contacting the cooled soot-lean syngas resulting from step (b) with water to remove HCN and NH3 resulting in clean syngas and HCN/NH3-rich waste water, wherein: (i) in step (a) the raw syngas is cooled below its dew point by its contact with water; (ii) the HCN/NH3-rich waste water resulting from step (c) is passed through at least one stripping column resulting in a clean water stream and a waste stream; and (iii) at least 50% (v/v) part of the clean water stream is recycled to step (c).
 2. The process according to claim 1, wherein a first part of the soot-rich waste water resulting from step (a) is cooled and re-used in step (a).
 3. The process according to claim 2, wherein a second part of the soot-rich waste water is cleaned before further treatment.
 4. The process according to claim 1, wherein at most 90% (v/v) of the clean water stream obtained from the HCN/NH3-rich waste water stream is recycled to step (c).
 5. The process according to claim 1, wherein at most 10% (v/v) of the clean water stream obtained from the HCN/NH3-rich waste water stream is recycled to step (a). 